Power Metal Oxide Semiconductor Field Effect Transistors (“MOSFET”) are a well known type of semiconductor transistor that may be used as a switching device in high power applications. A power MOSFET may be turned on or off by applying a gate bias voltage to a gate electrode of the device. When a power MOSFET is turned on (i.e., it is in its “on-state”), current is conducted through a channel of the MOSFET. When the bias voltage is removed from the gate electrode (or reduced below a threshold level), the current ceases to conduct through the channel. By way of example, an n-type MOSFET turns on when a gate bias voltage is applied that is sufficient to create a conductive n-type inversion layer in a p-type channel region of the device. This n-type inversion layer electrically connects the n-type source and drain regions of the MOSFET, thereby allowing for majority carrier conduction therebetween.
The gate electrode of a power MOSFET is separated from the channel region by a thin oxide gate insulating layer. Because the gate of the MOSFET is insulated from the channel region, minimal gate current is required to maintain the MOSFET in its on-state or to switch the MOSFET between its on-state and its off-state. The gate current is kept small during switching because the gate forms a capacitor with the channel region. Thus, only minimal charging and discharging current is required during switching, allowing for less complex gate drive circuitry. Moreover, because MOSFETS are unipolar devices in which current conduction occurs solely through majority carrier transport, MOSFETs may exhibit very high switching speeds. The drift region of a power MOSFET, however, may exhibit a relatively high on-resistance, which arises from the absence of minority carrier injection. This increased resistance can limit the forward current density achievable with power MOSFETs. Additionally, the gate insulating layer of MOSFETs may degrade over time with use of the MOSFET.
The bipolar junction transistor (“BJT”) is another well known type of semiconductor transistor that may be used as a switching device in high power applications. As is known to those of skill in the art, a BJT includes two p-n junctions that are formed in close proximity to each other in the semiconductor material. In operation, charge carriers enter a first region of the semiconductor material (which is called the emitter) that is adjacent one of the p-n junctions. Most of the charge carriers exit the device from a second region of the semiconductor material (which is called the collector) that is adjacent the other p-n junction. The collector and emitter are formed in regions of the semiconductor material that have the same conductivity type. A third, relatively thin region of the semiconductor material, known as the base, is positioned between the collector and the emitter and has a conductivity type that is opposite the conductivity type of the collector and the emitter. Thus, the two p-n junctions of the BJT are formed where the collector meets the base and where the base meets the emitter. By flowing a small current through the base of a BJT, a proportionally larger current passes from the emitter to the collector.
BJTs are current controlled devices in that a BJT is turned “on” (i.e., it is biased so that current flows between the emitter and the collector) by flowing a current through the base of the transistor. For example, in an NPN BJT (i.e., a BJT that has n-type collector and emitter regions and a p-type base region), the transistor is typically turned on by applying a positive voltage to the base to forward bias the base-emitter p-n junction. When the device is biased in this manner, holes flow into the base of the transistor where they are injected into the emitter. The holes are referred to as “majority carriers” because the base is a p-type region, and holes are the “normal” charge carriers in such a region. At the same time, electrons are injected from the emitter into the base, where they diffuse toward the collector. These electrons are referred to as “minority carriers” because electrons are not the normal charge carrier in the p-type base region. The device is referred to as a “bipolar” device because the emitter-collector current includes both electron and hole current.
A BJT may require a relatively large base current to maintain the device in its on-state. As such, relatively complex external drive circuits may be required to supply the relatively large base currents that can be required by high power BJTs. Moreover, the switching speeds of BJTs may be significantly slower than the switching speeds of power MOSFETs due to the bipolar nature of the current conduction.
Devices embodying a combination of bipolar current conduction with MOS-controlled current flow are also known. One example of such a device is the Insulated Gate Bipolar Transistor (“IGBT”), which is a device that combines the high impedance gate of the power MOSFET with the small on-state conduction losses of the power BJT. An IGBT may be implemented, for example, as a Darlington pair that includes a high voltage n-channel MOSFET at the input and a BJT at the output. The base current of the BJT is supplied through the channel of the MOSFET, thereby allowing a simplified external drive circuit. The IGBT may combine the high temperature, high current density switching characteristics of the BJT with the minimal drive requirement of the MOSFET.
Most power semiconductor devices are formed of silicon (“Si”), although a variety of other semiconductor materials have also been used. Silicon carbide (“SiC”) is one of these alternate materials. Silicon carbide has potentially advantageous semiconductor characteristics including, for example, a wide band-gap, high electric field breakdown strength, high thermal conductivity, high electron mobility, high melting point and high-saturated electron drift velocity. Thus, relative to devices formed in other semiconductor materials such as, for example, silicon, electronic devices formed in silicon carbide may have the capability of operating at higher temperatures, at high power densities, at higher speeds, at higher power levels and/or under high radiation densities.